The long term goal of this proposal is to gain insight into the cellular and molecular mechanisms underlying the generation of the specific high affinity autoantibodies to self antigens in patients with systemic autoimmune diseases. The B cell repertoire in these patients differ from that in healthy subjects with respect to the specificities of the autoantibodies produced and the phenotype and regulation of the producing cells. Owing to the high level of circulating antibodies binding self and foreign antigens,it has been proposed that in systemic autoimmune diseases the autoantibody response results from a process of polyclonal B cell activation. Our preliminary experiments suggested that in healthy subjects "autoantibodies" with such functional features can be produced by CD5+B lymphocytes, a discrete subset of the B cell repertoire. These "autoantibodies" are, mainly, IgM and bind a variety of antigens (polyreactive antibodies). The polyreactivity and low to moderate affinity of the specific monoclonal antibodies, in general IgG, produced by (CD5-) B cells we isolated from vaccinated subjects, and those of the monoclonal specific autoantibodies produced by B cells we isolated from autoimmune patients. We hypothesize that the production of high affinity autoantibodies in patients with systemic autoimmunity results from a process of specific B cell clonal selection and somatic point mutation driven by antigen. To test this hypothesis, we propose to study the contribution of CD5+ and CD%-B cells as well as the CD5-"sister" B cells we recently identified to the production of specific, possibly, pathogenic (rheumatoid factors and anti-DNA) autoantibodies in patients with systemic autoimmunity (rheumatoid arthritis and systemic lupus erythematosus, respectively), and to clone and sequence the variable gene segments utilized in these autoantibodies. Such cellular and molecular features will be compared with those we recently derived from the study of the high affinity antibody responses elicited by rabies virus and tetanus toxoid vaccines in normal humans. The proposed experiments should yield important information on the role of various B cell subsets, and variable antibody genes in shaping the profile of the human B cell repertoire. In addition, they should provide us with a variety of monoclonal antibody and gene tools essential to approach a systematic probing of the human B cell repertoire.